Fix Length mistake on Day4 py file

[Sananda-Dutta]

fixes #795
fixes #797

What i did

1. Making length of full_name variable to be 16 characters, instead of 15.(Issue 795)
2. Changing the challenge.isalpha() to be false.(Issue 797)

Screenshots

1st one((Issue 795):

2nd one(Issue 797):

Additional Context
Changes the both issue on same PR.

Thanks.